This invention relates to a magnetoresistive element and a magnetic recording apparatus, and more particularly, to a magnetoresistive element using a spin valve film including electron reflective layers on opposite surfaces of a non-magnetic intermediate layer, and a magnetic recording apparatus using this magnetoresistive element in its magnetic head.
Recently, for a higher density of HDD (hard disk drive), read elements with higher sensitivities are under development. Toward such purposes, researches are being made about elements using a giant magnetoresistance effect basically configured to interpose a non-magnetic conductive layer between two metal magnetic layers.
Hopeful one of those elements is an element having a structure called xe2x80x9cspin valvexe2x80x9d. This is an element interposing a non-magnetic layer between two metal ferromagnetic layers of which one ferromagnetic layer (called xe2x80x9cmagnetically fixed layerxe2x80x9d or xe2x80x9cpinned layerxe2x80x9d) is fixed with a bias magnetic field, or the like, whereas the other ferromagnetic layer, not magnetically fixed, (called xe2x80x9cmagnetically free layerxe2x80x9d or xe2x80x9cfree layerxe2x80x9d) reads out the magnetic field from the recording medium and changes its magnetic orientation relative to that of the former magnetic layer, thereby to obtain a giant magnetic resistance (see Phys. Rev. B. Vol. 45, 806(1992), J. Appl. Phys. Vol. 69, 4774(1991), etc.).
Researches are also being made about a model using a multi-layered structure that is construction repeat of a non-magnetic layer between metal ferromagnetic layers to enhance the magnetoresistance effect. In these models, however, interaction between magnetic layers is great, in general, and they invite some problems when used in HDD, such as insufficient sensitivity to a medium magnetic field.
Various improvements were proposed and made for obtaining higher outputs with these multi-layered structures. Among them, what is most effective is to thin the magnetically free layer and the non-magnetic intermediate layer. By thinning these films, shunt diversion can be reduced and output can be improved. There has been proposed a spin filter/spin valve structure attaining both output enhancement by thinning films and bias point adjustment in head designing, a spin valve film having a resistance changing ratio (MR) around 9% and a magnetic head compatible with HDD having the surface density of 25 Gbpsi (gigabit per square inch) have been obtained.
In this structure, however, the maximum MR is less than 10%, and it is insufficient for coping with future recording densities of 40 Gbpsi or even more. Therefore, researches are continued toward new structures for further improved MR.
Among those structures, specular spin valves (SPSV) are currently the most hopeful structure. They are configured to stack an electron specular reflective layer on a magnetic layer to thereby elongate the mean free path so as to accomplish a structure equivalent to a magnetic multi-layered film and increase the resistance-changing ratio. Concerning this type of structure, heretofore reported are electron specular reflectance effects with oxide antiferromagnetic materials such NiO used as exchange bias films of magnetically pinned layers, amorphous Ta formed as a base layer of a magnetically free layer, or noble metal layers. There is also reported that MR is improved by electron reflection when oxide layers on the order of nanometer are formed in a magnetically pinned layer and a magnetically free layer (IEEE Trans. Mag., Vol. 33, No. 5, p3580, 9(1997), IEEE Trans. Mag., Vol. 32, No. 5, p4728, 9(1996)).
Among those conventional structures, those using an oxide antiferromagnetic material such as NiO as the exchange bias film involve the problem of a small exchange coupling force due to the natures peculiar to the materials. Additionally, when introducing a magnetically pinned layer having a synthetic antiferromagnetic structure, which is currently an indispensable structural item, electron reflection from the oxide exchange bias layer no longer contributes to MR, and the merit of MR improvement is lost.
Further, Ru (ruthenium) used in the coupling layer of a synthetic antiferromagnetic structure is a substance providing no electron reflecting effect. On the other hand, if amorphous Ta (tantalum) is used, it adversely affects good growth of films, and invites deterioration of soft-magnetic characteristics of the magnetically free layer, thermal stability of MR, thermal stability of magnetic pinning of the magnetically pinned layer and synthetic antiferromagnetic coupling.
Furthermore, if a noble metal or amorphous Ta layer is formed on the part of the magnetically pinned layer, magnetic coupling between layers on opposite sides thereof will be disconnected, and it is therefore inappropriate as a technique for obtaining electron-reflecting effects. For the purpose of maximizing MR, it is desirable that electron reflection occurs both in the magnetically free layer and the magnetically pinned layer.
The above-mentioned problem will be overcome with a method of making magnetic oxide layers of a thickness on the order of nanometer in the magnetically pinned layer and the magnetically free layer. This method, however, involves the problems, such as the need for a technically high-level process for making the oxide layer of a thickness on the order of nanometer, the need for a special processing apparatus, and the need for a time for fabrication.
As explained above, it is necessary for maximizing MR to make electron reflecting layers in both the magnetically free layer and the magnetically pinned layer, but the method of obtaining an electron reflection effect on the part of the magnetically pinned layer is currently only the method of making an oxide layer on the order of nanometer, and involved the problem that a high-level technique is required to make it.
The present invention has been made from recognition of those problems. It is therefore an object of the invention to provide a structure of a magnetically pinned layer ensuring a sufficient electron reflection effect on the part of the magnetically pinned layer, and a magnetoresistive element using a pin valve film including the structure.
In order to attain the object, a magnetoresistive element according to the invention includes a magnetically free layer, a magnetically pinned layer, and a non-magnetic intermediate layer interposed between the magnetically free layer and the magnetically pinned layer, wherein the magnetically pinned layer includes, at least, a first layer region disposed relatively remoter from the non-magnetic intermediate layer and a second layer region disposed relatively closer to the non-magnetic intermediate layer, and the first layer region is made of a ferromagnetic material containing a special additive element.
The additive element is one that can effectively change the electron potential of the magnetically pinned layer and can simultaneously maintain the crystal structure of the magnetically pinned layer. Usable as its additive element is any one selected from Cr (chrome), Rh (rhodium), Os (osmium), Re (rhenium), Si (silicon), Al (aluminum), Be (beryllium), Ga (gallium), Ge (germanium), Te (tellurium), B (boron), V (vanadium), Ru (ruthenium), Ir (iridium), W (tungsten), Mo (molybdenum), Au (gold), Pt (platinum), Ag (silver) and Cu (copper).
By addition of one or more of these elements, the magnetically pinned layer can be changed in electron potential, and electron reflection can be brought about along the boundary. Nb (niobium), Ta (tantalum), or other like materials, are not suitable as the additive elements because they change the crystal to microcrystal. Change of the crystal to microcrystal or amorphous form will adversely affects the crystallographic property of the film grown thereon.
For example, in case of a bottom spin valve having the magnetically free layer disposed above the magnetically pinned layer, since the crystalline property and orientation of the film formed above the first layer region to function as the electron reflecting layer are deteriorated, the soft-magnetic property of the magnetically free layer is deteriorated. More specifically, from the viewpoint of the MR merit, the magnetically free layer is preferably a Co alloy, but in this case, the fcc-Co alloy has to be (111)-oriented to ensure a soft-magnetic property. Therefore, the first layer region serving as the electron-reflecting layer is preferably of a crystal. Further, In a fcc (face centered cubit) or hcp (hexagonal closest packed) structure, the closest-packed atomic plane is preferably oriented in parallel to the major surface of the layer, that is, the film surface, and more preferably fcc(111)-oriented. Additionally, deterioration of the crystalline property will cause atomic diffusion and will invite deterioration of MR.
For example, in case of a top spin valve in which the magnetically free layer is disposed below the magnetically pinned layer on a substrate, the film formed on the first layer region serving as the electron reflecting layer will deteriorates in crystalline property and orientation, and therefore, the antiferromagnetic coupling in the synthetic antiferromagnetic structure will weaken. Further, since deterioration of the crystalline property causes atomic diffusion, it not only deteriorates MR but also causes the phenomenon that an Ru layer having a thickness around 1 nm, which is normally used for making the synthetic antiferromagnetic structure, diffuses and eventually destructs the synthetic antiferromagnetic structure.
From those reasons, the additive element for increasing the electron potential is desired not to change the crystalline property. Any of the above-indicated additive elements does not change the crystallographic property even when added to a crystalline ferromagnetic alloy, and does not deteriorate the quality of the film formed thereon. Moreover, when one of Cr, Rh, Os, Re, Si, Al, Be, Ga and Ge, in particular, is used among those proposed additive elements, good crystalline quality will be obtained.
It is important, however, that the ratio of the additive element must be within a value that does not remove the magnetism of the original ferromagnetic metal. Once the layer loses magnetism, it will no longer function as the magnetically pinned layer. For example, when the matrix material of the magnetically pinned layer is Co90Fe10, the amount of Cr added is 20% or more in atomic %, the ferromagnetic property will become instable. Therefore, the amount to be added should be limited to 20% or less. Preferably, its atomic percent is desired to be 15% or less. On the other hand, to ensure a sufficiently large potential difference, a certain ratio should be contained. An amount not less than 3% is preferable.
As the thickness increases, the first layer region added with the additive element is liable to cause a decrease of the output by shunt diversion, and it should be thinned previously. Through researches, the Inventor has found that desirable thickness of the first layer would be 2 nm or less. However, when the thickness decreases to a certain value, electron reflection effect will be lost due to atomic diffusion. From this viewpoint, the thickness is preferably determined 0.5 nm or more.
In any of those cases, all layers not contacting the non-magnetic intermediate layer among those magnetically pinned layers, are desirably raised in resistance because they do not contribute to MR effects but cause an output decreases by shunt diversion. Simultaneously, these layers are also desirable to maintain good crystalline properties in order to stabilize the magnetically pinned layer. For this point of view, the additive element for enhancing the resistance values of these layers should preferably be at least one selected from Cr, Rh, Os, Re, Si, Al, Be, Ga, Ge, Te, B, V, Ru, Ir, W, Mo, Au, Pt, Ag and Cu.
On the other hand, the magnetic recording apparatus according to the invention is characterized in including a magnetic head for writing or reading information on or from a magnetic recording medium, which magnetic head uses one of the above-summarized magnetoresistive elements, and ensures magnetic writing and reading with much higher density than conventional ones.
As summarized above, by using the invention, it is possible to make an electron-reflecting layer of a metal in the magnetically pinned layer without losing the synthetic ferromagnetic property and the soft-magnetic property of the magnetically free layer. As a result, it is possible to significantly enhance the output of the magnetoresistive element and realize a magnetic writing/reading system with a much higher recording density than conventional ones. Thus the industrial merit of the invention is great.